The development of modern electronic devices places an ever increasing demand on the quality of the semiconductor materials used in these devices. However, the epitaxially grown semiconductor crystals produced with today's technology are highly defective and adversely affect the performance and reliability of the device employing the crystal.
The poor quality of the epitaxially grown semiconductor crystals is generally due to the unavailability of a lattice matched and chemically matched substrate. When a film is grown on a substrate or an underlying layer such that the crystal structure of the substrate or underlying layer is reflected on the film, the thus-grown film exhibits microstructural discontinuity at the interface between layers formed of different semiconductor materials. Since the thickness of the film is considerably smaller than that of the substrate, the difference in lattice constant between the film and the substrate causes generation of strain and defects in the film.
For example, when a crystal of a nitride semiconductor such as gallium nitride (GaN) is grown on a starting substrate made of silicon (Si) or the like and then cooled to the ordinary temperature, a large number of dislocations and cracks occur in the nitride semiconductor layer because of stress caused by the difference in thermal expansion coefficient or lattice constant. If a large number of dislocations and cracks occur in the growth layer (nitride semiconductor layer), lattice defects or a large number of dislocations, deformations, cracks, etc., occur in a film when the film is epitaxially formed on the growth layer. This causes deterioration of the film characteristic.
To improve lattice match, a buffer layer can be formed on a substrate, and subsequently a crystal of a semiconductor is grown on the buffer layer. The buffer layer can lower the adverse effect of defects in the surface of the substrate on the crystal, and reduce propagation of lattice defects contained in the substrate. However, in some cases, because of microstructural discontinuity at the substrate/buffer interface, defects are newly generated at the interface.
Therefore, demand has arisen for an epitaxially grown compound semiconductor film of reduced crystal defect density.